fix: resolve schemathesis and live test failures

- Add 403 response to all RBAC-gated endpoints (schemathesis UndefinedStatusCode)
- Add 400 response to all endpoints accepting JSON bodies (malformed input)
- Add required 'title' field to schemathesis.toml for schemathesis 4.15+
- Add xdist_group markers to live tests with module-scoped fixtures to
  prevent xdist from distributing them across workers (fixture isolation)

decnet/web/router/fleet/api_get_deckies.py

@@ -9,7 +9,7 @@ router = APIRouter()
 
 
 @router.get("/deckies", tags=["Fleet Management"],
-    responses={401: {"description": "Could not validate credentials"}, 422: {"description": "Validation error"}},)
+    responses={401: {"description": "Could not validate credentials"}, 403: {"description": "Insufficient permissions"}, 422: {"description": "Validation error"}},)
 @_traced("api.get_deckies")
 async def get_deckies(user: dict = Depends(require_viewer)) -> list[dict[str, Any]]:
     return await repo.get_deckies()